Open-circuit voltage improvement in tantalum-doped TiO2 nanocrystalsElectronic supplementary information (ESI) available: Detailed experimental section, DSSC device assembly and characterization, XRD patterns, additional SEM, TEM pictures, XPS results and photographs of powders heat-treated in different atmosphere. See DOI: 10.1039/c4cp01655f

Enhanced electron concentration derived from Ta 5+ doping is responsible for the open-circuit voltage improvement due to the upward shift of the Fermi level, but the oxygen defects generated retard the negative shift of the Fermi level. By mediating the trap states, highly efficient DSSC devices could be achieved. Enhanced electron concentration is responsible for the open-circuit voltage improvement due to the upward shift of the Fermi level.